Somatostatin increases voltage-dependent potassium currents in rat somatotrophs

Beta cell primary cilia mediate somatostatin responsiveness via SSTR3

Somatostatin is a paracrine modulator of insulin secretion and beta cell function with pleotropic effects on glucose homeostasis. The mechanism of somatostatin-mediated communication between delta and beta cells is not well-understood, which we address in this study via the ciliary somatostatin receptor 3 (SSTR3). Primary cilia are membrane organelles that act as signaling hubs in islets by virtue of their subcellular location and enrichment in signaling proteins such as G-protein coupled receptors (GPCRs). We show that SSTR3, a ciliary GPCR, mediates somatostatin suppression of insulin secretion in mouse islets. Quantitative analysis of calcium flux using a mouse model of genetically encoded beta cell-specific GCaMP6f calcium reporter shows that somatostatin signaling alters beta cell calcium flux after physiologic glucose stimulation, an effect that depends on endogenous SSTR3 expression and the presence of intact primary cilia on beta cells. Comparative in vitro studies using SSTR isoform antagonists demonstrate a role for SSTR3 in mediating somatostatin regulation of insulin secretion in mouse islets. Our findings support a model in which ciliary SSTR3 mediates a distinct pathway of delta-to-beta cell regulatory crosstalk and may serve as a target for paracrine modulation.

Neuropeptides and small-molecule amine transmitters: cooperative signaling in the nervous system

Neuropeptides are expressed in cell-specific patterns throughout mammalian brain. Neuropeptide gene expression has been useful for clustering neurons by phenotype, based on single-cell transcriptomics, and for defining specific functional circuits throughout the brain. How neuropeptides function as first messengers in inter-neuronal communication, in cooperation with classical small-molecule amine transmitters (SMATs) is a current topic of systems neurobiology. Questions include how neuropeptides and SMATs cooperate in neurotransmission at the molecular, cellular and circuit levels; whether neuropeptides and SMATs always co-exist in neurons; where neuropeptides and SMATs are stored in the neuron, released from the neuron and acting, and at which receptors, after release; and how neuropeptides affect 'classical' transmitter function, both directly upon co-release, and indirectly, via long-term regulation of gene transcription and neuronal plasticity. Here, we review an extensive body of data about the distribution of neuropeptides and their receptors, their actions after neuronal release, and their function based on pharmacological and genetic loss- and gain-of-function experiments, that addresses these questions, fundamental to understanding brain function, and development of neuropeptide-based, and potentially combinatorial peptide/SMAT-based, neurotherapeutics.

Common and diverse elements of ion channels and receptors underlying electrical activity in endocrine pituitary cells

The pituitary gland contains six types of endocrine cells defined by hormones they secrete: corticotrophs, melanotrophs, gonadotrophs, thyrotrophs, somatotrophs, and lactotrophs. All these cell types are electrically excitable, and voltage-gated calcium influx is the major trigger for their hormone secretion. Along with hormone intracellular content, G-protein-coupled receptor and ion channel expression can also be considered as defining cell type identity. While many aspects of the developmental and activity dependent regulation of hormone and G-protein-coupled receptor expression have been elucidated, much less is known about the regulation of the ion channels needed for excitation-secretion coupling in these cells. We compare the spontaneous and receptor-controlled patterns of electrical signaling among endocrine pituitary cell types, including insights gained from mathematical modeling. We argue that a common set of ionic currents unites these cells, while differential expression of another subset of ionic currents could underlie cell type-specific patterns. We demonstrate these ideas using a generic mathematical model, showing that it reproduces many observed features of pituitary electrical signaling. Mapping these observations to the developmental lineage suggests possible modes of regulation that may give rise to mature pituitary cell types.

Ion Channels of Pituitary Gonadotrophs and Their Roles in Signaling and Secretion

Gonadotrophs are basophilic cells of the anterior pituitary gland specialized to secrete gonadotropins in response to elevation in intracellular calcium concentration. These cells fire action potentials (APs) spontaneously, coupled with voltage-gated calcium influx of insufficient amplitude to trigger gonadotropin release. The spontaneous excitability of gonadotrophs reflects the expression of voltage-gated sodium, calcium, potassium, non-selective cation-conducting, and chloride channels at their plasma membrane (PM). These cells also express the hyperpolarization-activated and cyclic nucleotide-gated cation channels at the PM, as well as GABA_A, nicotinic, and purinergic P2X channels gated by γ-aminobutyric acid (GABA), acetylcholine (ACh), and ATP, respectively. Activation of these channels leads to initiation or amplification of the pacemaking activity, facilitation of calcium influx, and activation of the exocytic pathway. Gonadotrophs also express calcium-conducting channels at the endoplasmic reticulum membranes gated by inositol trisphosphate and intracellular calcium. These channels are activated potently by hypothalamic gonadotropin-releasing hormone (GnRH) and less potently by several paracrine calcium-mobilizing agonists, including pituitary adenylate cyclase-activating peptides, endothelins, ACh, vasopressin, and oxytocin. Activation of these channels causes oscillatory calcium release and a rapid gonadotropin release, accompanied with a shift from tonic firing of single APs to periodic bursting type of electrical activity, which accounts for a sustained calcium signaling and gonadotropin secretion. This review summarizes our current understanding of ion channels as signaling molecules in gonadotrophs, the role of GnRH and paracrine agonists in their gating, and the cross talk among channels.

Potassium Current Is Not Affected by Long-Term Exposure to Ghrelin or GHRP-6 in Somatotropes GC Cells

Ghrelin is a growth hormone (GH) secretagogue (GHS) and GHRP-6 is a synthetic peptide analogue; both act through the GHS receptor. GH secretion depends directly on the intracellular concentration of Ca(2+); this is determined from the intracellular reserves and by the entrance of Ca(2+) through the voltage-dependent calcium channels, which are activated by the membrane depolarization. Membrane potential is mainly determined by K(+) channels. In the present work, we investigated the effect of ghrelin (10 nM) or GHRP-6 (100 nM) for 96 h on functional expression of voltage-dependent K(+) channels in rat somatotropes: GC cell line. Physiological patch-clamp whole-cell recording was used to register the K(+) currents. With Cd(2+) (1 mM) and tetrodotoxin (1  μ m) in the bath solution recording, three types of currents were characterized on the basis of their biophysical and pharmacological properties. GC cells showed a K(+) current with a transitory component (I A) sensitive to 4-aminopyridine, which represents ~40% of the total outgoing current; a sustained component named delayed rectifier (I K), sensitive to tetraethylammonium; and a third type of K(+) current was recorded at potentials more negative than -80 mV, permitting the entrance of K(+) named inward rectifier (KIR). Chronic treatment with ghrelin or GHRP-6 did not modify the functional expression of K(+) channels, without significant changes (P < 0.05) in the amplitudes of the three currents observed; in addition, there were no modifications in their biophysical properties and kinetic activation or inactivation.

Ion channels and signaling in the pituitary gland

Endocrine pituitary cells are neuronlike; they express numerous voltage-gated sodium, calcium, potassium, and chloride channels and fire action potentials spontaneously, accompanied by a rise in intracellular calcium. In some cells, spontaneous electrical activity is sufficient to drive the intracellular calcium concentration above the threshold for stimulus-secretion and stimulus-transcription coupling. In others, the function of these action potentials is to maintain the cells in a responsive state with cytosolic calcium near, but below, the threshold level. Some pituitary cells also express gap junction channels, which could be used for intercellular Ca(2+) signaling in these cells. Endocrine cells also express extracellular ligand-gated ion channels, and their activation by hypothalamic and intrapituitary hormones leads to amplification of the pacemaking activity and facilitation of calcium influx and hormone release. These cells also express numerous G protein-coupled receptors, which can stimulate or silence electrical activity and action potential-dependent calcium influx and hormone release. Other members of this receptor family can activate calcium channels in the endoplasmic reticulum, leading to a cell type-specific modulation of electrical activity. This review summarizes recent findings in this field and our current understanding of the complex relationship between voltage-gated ion channels, ligand-gated ion channels, gap junction channels, and G protein-coupled receptors in pituitary cells.

Inhibitory effect of somatostatin-14 on L-type voltage-gated calcium channels in cultured cone photoreceptors requires intracellular calcium

The inhibitory effects of somatostatin have been well documented for many physiological processes. The action of somatostatin is through G-protein-coupled receptor-mediated second-messenger signaling, which in turn affects other downstream targets including ion channels. In the retina, somatostatin is released from a specific class of amacrine cells. Here we report that there was a circadian phase-dependent effect of somatostatin-14 (SS14) on the L-type voltage-gated calcium channels (L-VGCCs) in cultured chicken cone photoreceptors, and our study reveals that this process is dependent on intracellular calcium stores. Application of 500 nM SS14 for 2 h caused a decrease in L-VGCC currents only during the subjective night but not the subjective day. We then explored the cellular mechanisms underlying the circadian phase-dependent effect of SS14. The inhibitory effect of SS14 on L-VGCCs was mediated through the pertussis-toxin-sensitive G-protein-dependent somatostatin receptor 2 (sst2). Activation of sst2 by SS14 further activated downstream signaling involving phospholipase C and intracellular calcium stores. Mobilization of intracellular Ca2+ was required for somatostatin induced inhibition of photoreceptor L-VGCCs, suggesting that somatostatin plays an important role in the modulation of photoreceptor physiology.

Involvement of somatostatin receptor subtypes in membrane ion channel modification by somatostatin in pituitary somatotropes

1. Growth hormone (GH) secretion from pituitary somatotropes is mainly regulated by two hypothalamic hormones, GH-releasing hormone (GHRH) and somatotrophin releasing inhibitory factor (SRIF). 2. Somatotrophin releasing inhibitory factor inhibits GH secretion via activation of specific membrane receptors, somatostatin receptors (SSTRs) and signalling transduction systems in somatotropes. 3. Five subtypes of SSTRs, namely SSTR1, 2, 3, 4 and 5, have been identified, with the SSTR2 subtype divided into SSTR2A and SSTR2B. All SSTRs are G-protein-coupled receptors. 4. Voltage-gated Ca(2+) and K(+) channels on the somatotrope membrane play an important role in regulating GH secretion and SRIF modifies both channels to reduce intracellular free Ca(2+) concentration and GH secretion. 5. Using specific SSTR subtype-specific agonists, it has been found that reduction in Ca(2+) currents by SRIF is mediated by SSTR2 and an increase in K(+) currents is mediated by both SSTR2 and SSTR4 in rat somatotropes.

Mechanism of Spontaneous and Receptor-Controlled Electrical Activity in Pituitary Somatotrophs: Experiments and Theory

Cultured pituitary somatotrophs release growth hormone in response to spontaneous Ca2+ entry through voltage-gated calcium channels (VGCCs) that is governed by plateau-bursting electrical activity and is regulated by several neurohormones, including GH-releasing hormone (GHRH) and somatostatin. Here we combine experiments and theory to clarify the mechanisms underlying spontaneous and receptor-controlled electrical activity. Experiments support a role of a Na+-conducting and tetrodotoxin-insensitive channel in controlling spontaneous and GHRH-stimulated pacemaking, the latter in a cAMP-dependent manner; an opposing role of spontaneously active inwardly rectifying K+ ( Kir) channels and G-protein-regulated Kir channels in somatostatin-mediated inhibition of pacemaking; as well as a role of VGCCs in spiking and large conductance (BK-type) Ca2+-activated K+ channels in plateau bursting. The mathematical model is compatible with a wide variety of experimental data involving pharmacology and extracellular ion substitution and supports the importance of constitutively active tetrodotoxin-insensitive Na+ and Kir channels in maintaining spontaneous pacemaking in pituitary somatotrophs. The model also suggests that these channels are involved in the up- and downregulation of electrical activity by GHRH and somatostatin. In the model, the plateau bursting is controlled by two functional populations of BK channels, characterized by distance from the VGCCs. The rapid activation of the proximal BK channels is critical for the establishment of the plateau, whereas slow recruitment of the distal BK channels terminates the plateau.

Regulation of AQP2 in Collecting Duct : An emphasis on the Effects of Angiotensin II or Aldosterone

Vasopressin, angiotensin II (AngII), and aldosterone are essential hormones in the regulation of body fluid homeostatsis. We examined the effects of AngII or aldosterone on the regulation of body water balance. We demonstrated that 1) short-term treatment with AngII in the primary cultured inner medullary collecting duct cells played a role in the regulation of AQP2 targeting to the plasma membrane through AT1 receptor activation. This potentiated the effects of dDAVP on cAMP accumulation, AQP2 phosphorylation, and AQP2 plasma membrane targeting; 2) pharmacological blockade of the AngII AT1 receptor in rats co-treated with dDAVP and dietary NaCl-restriction (to induce high plasma endogenous AngII) resulted in an increase in urine production, a decrease in urine osmolality, and blunted the dDAVP-induced upregulation of AQP2; 3) long-term aldosterone infusion in normal rats or in rats with diabetes insipidus was associated with polyuria and decreased urine concentration, accompanied by decreased apical but increased basolateral AQP2 labeling intensity in the connecting tubule and cortical collecting duct; and 4) in contrast to the effects of dDAVP and AngII, short-term aldosterone treatment does not alter the intracellular distribution of AQP2. In conclusion, angiotensin II, and aldosterone could play a role in the regulation of renal water reabsorption by changing intracellular AQP2 targeting and/or AQP2 abundance, in addition to the vasopressin.

Postoperative pulmonary complications

PURPOSE OF REVIEW

Postoperative pulmonary complications, including pneumonia, bronchospasm, respiratory failure and prolonged mechanical ventilation, occur commonly and are a significant source of morbidity and mortality. This review will discuss the etiology of postoperative pulmonary complications and the interventions that reduce their risk.

RECENT FINDINGS

General anesthesia and surgery produce changes in the respiratory system and are responsible, along with underlying conditions, for postoperative pulmonary complications. Risk factors include upper abdominal or thoracic surgery, cigarette smoking, chronic respiratory disease, emergency surgery, anesthetic time of 180 min or more, age greater than 70 years, renal failure, poor nutritional status, and significant intraoperative blood loss. The inhibition of phrenic nerve output results in postoperative diaphragmatic dysfunction. Sleep-disordered breathing occurs after surgery even in patients without obstructive sleep apnea, but patients with obstructive sleep apnea may have a worsening of their disease after surgery. A clear advantage of one anesthetic technique over another in reducing postoperative pulmonary complications has not been demonstrated. Conflicting results have been obtained regarding the value of epidural analgesia in preventing postoperative pulmonary complications. Incentive spirometry decreases rates of postoperative pulmonary complications and hospital lengths of stay.

SUMMARY

Understanding risk factors for the development of postoperative pulmonary complications allows targeted interventions aimed at reducing their frequency and severity. Further research is needed to define the role of regional analgesic and anesthetic techniques in reducing postoperative pulmonary complications, and also to define the nature of risk factors and develop better predictive models of patients at risk of developing postoperative pulmonary complications.

Reduction in voltage-gated K+ currents in primary cultured rat pancreatic beta-cells by linoleic acids

Free fatty acids (FFAs), in addition to glucose, have been shown to stimulate insulin release through the G protein-coupled receptor (GPCR)40 receptor in pancreatic beta-cells. Intracellular free calcium concentration ([Ca(2+)](i)) in beta-cells is elevated by FFAs, although the mechanism underlying the [Ca(2+)](i) increase is still unknown. In this study, we investigated the action of linoleic acid on voltage-gated K(+) currents. Nystatin-perforated recordings were performed on identified rat beta-cells. In the presence of nifedipine, tetrodotoxin, and tolbutamide, voltage-gated K(+) currents were observed. The transient current represents less than 5%, whereas the delayed rectifier current comprises more than 95%, of the total K(+) currents. A long-chain unsaturated FFA, linoleic acid (10 microm), reversibly decreased the amplitude of K(+) currents (to less than 10%). This reduction was abolished by the cAMP/protein kinase A system inhibitors H89 (1 microm) and Rp-cAMP (10 microm) but was not affected by protein kinase C inhibitor. In addition, forskolin and 8'-bromo-cAMP induced a similar reduction in the K(+) current as that evoked by linoleic acid. Insulin secretion and cAMP accumulation in beta-cells were also increased by linoleic acid. Methyl linoleate, which has a similar structure to linoleic acid but no binding affinity to GPR40, did not change K(+) currents. Treatment of cultured cells with GPR40-specific small interfering RNA significantly reduced the decrease in K(+) current induced by linoleic acid, whereas the cAMP-induced reduction of K(+) current was not affected. We conclude that linoleic acid reduces the voltage-gated K(+) current in rat beta-cells through GPR40 and the cAMP-protein kinase A system, leading to an increase in [Ca(2+)](i) and insulin secretion.

Somatostatin increases voltage-gated K+ currents in GH3 cells through activation of multiple somatostatin receptors

The secretion of GH by somatotropes is inhibited by somatostatin (SRIF) through five specific membrane receptors (SSTRs). SRIF increases both transient outward (IA) and delayed rectifying (IK) K+ currents. We aim to clarify the subtype(s) of SSTRs involved in K+ current enhancement in GH3 somatotrope cells using specific SSTR subtype agonists. Expression of all five SSTRs was confirmed in GH3 cells by RT-PCR. Nystatin-perforated patch clamp was used to record voltage-gated K+ currents. We first established the presence of IA and IK type K+ currents in GH3 cells using different holding potentials (-40 or -70 mV) and specific blockers (4-aminopirimidine and tetraethylammonium chloride). SRIF (200 nM) increased the amplitude of both IA and IK in a fully reversible manner. Various concentrations of each specific SRTR agonist were tested on K+ currents to find the maximal effective concentration. Activation of SSTR2 and SSTR4 by their respective agonists, L-779,976 and L-803,087 (10 nM), increased K+ current amplitude without preference to IA or IK, and abolished any further increase by SRIF. Activation of SSTR1 and SSTR5 by their respective agonists, L-797,591 or L-817,818 (10 nM), increased K+ current amplitude, but SRIF evoked a further increase. The SSTR3 agonist L-797,778 (10 nM) did not affect the K+ currents or the response to SRIF. These results indicate that SSTR1, -2, -4, and -5 may all be involved in the enhancement of K+ currents by SRIF but that only the activation of SSTR2 or -4 results in the full activation of K+ current caused by SRIF.

Ghrelin reduces voltage-gated potassium currents in GH3 cells via cyclic GMP pathways

Ghrelin is an endogenous growth hormone secretagogue (GHS) causing release of GH from pituitary somatotropes through the GHS receptor. Secretion of GH is linked directly to intracellular free Ca(2+) concentration ([Ca(2+)]i), which is determined by Ca(2+) influx and release from intracellular Ca(2+) storage sites. Ca(2+) influx is via voltage-gated Ca(2+) channels, which are activated by cell depolarization. Membrane potential is mainly determined by transmembrane K(+) channels. The present study investigates the in vitroeffect of ghrelin on membrane voltage-gated K(+) channels in the GH3 rat somatotrope cell line. Nystatin-perforated patch clamp recording was used to record K(+) currents under voltage-clamp conditions. In the presence of Co(2+) (1 mM, Ca(2+) channel blocker) and tetrodotoxin (1 microM, Na(+) channel blocker) in the bath solution, two types of voltage-gated K(+) currents were characterized on the basis of their biophysical kinetics and pharmacological properties. We observed that transient K(+) current (IA) represented a significant proportion of total K(+) currents in some cells, whereas delayed rectifier K(+) current (IK) existed in all cells. The application of ghrelin (10 nM) reversibly and significantly decreased the amplitude of both IA and IK currents to 48% and 64% of control, respectively. Application of apamin (1 microM, SK channel blocker) or charybdotoxin (1 microM, BK channel blocker) did not alter the K(+) current or the response to ghrelin. The ghrelin-induced reduction in K(+) currents was not affected by PKC and PKA inhibitors. KT5823, a specific PKG inhibitor, totally abolished the K+ current response to ghrelin. These results suggest that ghrelin-induced reduction of voltage-gated K(+) currents in GH3 cells is mediated through a PKG-dependent pathway. A decrease in voltage-gated K(+) currents may increase the frequency, duration, and amplitude of action potentials and contribute to GH secretion from somatotropes.

Estimation of parameters for a mathematical model of growth hormone secretion

Here, we describe partial calibration of a parsimonious mathematical model of growth hormone (GH) secretion. From first principles, we derived a model of the effects on GH secretion from pituitary somatotrophs of stimulation by GH-releasing factor (GRF) or GH secretagogue, and of inhibition by somatostatin. We obtained a concise model by collapsing the many processes of the signal transduction cascade into a single step broadly reflecting the initial binding of GRF to its receptors. In the model, GH secretion is proportional to the rate of binding of GRF to activatable receptors. Desensitization occurs because of reduction of free receptors/available effector units, and resensitization occurs as those lost are replaced. This replacement is speeded up in the presence of somatostatin, which also inhibits GH secretion by reducing the constant of proportionality between the rate of GH secretion and the rate of GRF binding. We derived simple mathematical equations for the rate of GH secretion and cumulative secretion. Using these, we tested the model against data obtained from experiments performed in vitro, and made it quantitative using rigorous statistical approaches to optimize parameter estimates. The behaviour of the calibrated model matches experimental observations closely.

Somatostatin receptors

In 1972, Brazeau et al. isolated somatostatin (somatotropin release-inhibiting factor, SRIF), a cyclic polypeptide with two biologically active isoforms (SRIF-14 and SRIF-28). This event prompted the successful quest for SRIF receptors. Then, nearly a quarter of a century later, it was announced that a neuropeptide, to be named cortistatin (CST), had been cloned, bearing strong resemblance to SRIF. Evidence of special CST receptors never emerged, however. CST rather competed with both SRIF isoforms for specific receptor binding. And binding to the known subtypes with affinities in the nanomolar range, it has therefore been acknowledged to be a third endogenous ligand at SRIF receptors. This review goes through mechanisms of signal transduction, pharmacology, and anatomical distribution of SRIF receptors. Structurally, SRIF receptors belong to the superfamily of G protein-coupled (GPC) receptors, sharing the characteristic seven-transmembrane-segment (STMS) topography. Years of intensive research have resulted in cloning of five receptor subtypes (sst(1)-sst(5)), one of which is represented by two splice variants (sst(2A) and sst(2B)). The individual subtypes, functionally coupled to the effectors of signal transduction, are differentially expressed throughout the mammalian organism, with corresponding differences in physiological impact. It is evident that receptor function, from a physiological point of view, cannot simply be reduced to the accumulated operations of individual receptors. Far from being isolated functional units, receptors co-operate. The total receptor apparatus of individual cell types is composed of different-ligand receptors (e.g. SRIF and non-SRIF receptors) and co-expressed receptor subtypes (e.g. sst(2) and sst(5) receptors) in characteristic proportions. In other words, levels of individual receptor subtypes are highly cell-specific and vary with the co-expression of different-ligand receptors. However, the question is how to quantify the relative contributions of individual receptor subtypes to the integration of transduced signals, ultimately the result of collective receptor activity. The generation of knock-out (KO) mice, intended as a means to define the contributions made by individual receptor subtypes, necessarily marks but an approximation. Furthermore, we must now take into account the stunning complexity of receptor co-operation indicated by the observation of receptor homo- and heterodimerisation, let alone oligomerisation. Theoretically, this phenomenon adds a novel series of functional megareceptors/super-receptors, with varied pharmacological profiles, to the catalogue of monomeric receptor subtypes isolated and cloned in the past. SRIF analogues include both peptides and non-peptides, receptor agonists and antagonists. Relatively long half lives, as compared to those of the endogenous ligands, have been paramount from the outset. Motivated by theoretical puzzles or the shortcomings of present-day diagnostics and therapy, investigators have also aimed to produce subtype-selective analogues. Several have become available.

A novel vasopressin-induced transcript promotes MAP kinase activation and ENaC downregulation

In the principal cell of the renal collecting duct, vasopressin regulates the expression of a gene network responsible for sodium and water reabsorption through the regulation of the water channel and the epithelial sodium channel (ENaC). We have recently identified a novel vasopressin-induced transcript (VIT32) that encodes for a 142 amino acid vasopressin-induced protein (VIP32), which has no homology with any protein of known function. The Xenopus oocyte expression system revealed two functions: (i) when injected alone, VIT32 cRNA rapidly induces oocyte meiotic maturation through the activation of the maturation promoting factor, the amphibian homolog of the universal M phase trigger Cdc2/cyclin; and (ii) when co-injected with the ENaC, VIT32 cRNA selectively downregulates channel activity, but not channel cell surface expression. In the kidney principal cell, VIP32 may be involved in the downregulation of transepithelial sodium transport observed within a few hours after vasopressin treatment. VIP32 belongs to a novel gene family ubiquitously expressed in oocyte and somatic cells that may be involved in G to M transition and cell cycling.

Calcyclin is an early vasopressin-induced gene in the renal collecting duct. Role in the long term regulation of ion transport

Long-term effects of arginine vasopressin (AVP) in the kidney involve the transcription of unidentified genes. By subtractive hybridization experiments performed on the RCCD(1) cortical collecting duct cell line, we identified calcyclin as an early AVP-induced gene (1 h). Calcyclin is a calcium-binding protein involved in the transduction of intracellular signals. In the kidney, calcyclin was localized at the mRNA level in the glomerulus, all along the collecting duct, and in the epithelium lining the papilla. In RCCD(1) cells and in m-IMCD(3) inner medullary collecting duct cells, calcyclin was evidenced in the cytoplasm. Calcyclin mRNA levels were progressively increased by AVP treatment in RCCD(1) (1.7-fold at 4 h) and m-IMCD(3) (2-fold at 7.5 h) cells. In RCCD(1) cells, calcyclin protein levels were increased by 4 h of AVP treatment. In vivo, treatment of genetically vasopressin-deficient Brattleboro rats with AVP for 4 days induced an increase in both calcyclin and aquaporin-2 mRNA expression. Finally, introduction of anti-calcyclin antibodies into RCCD(1) cells by permeabilizing the plasma membrane prevented the long-term (but not short-term) increase in short-circuit current induced by AVP. Taken together, these results suggest that calcyclin is an early vasopressin-induced gene that participates in the late phase of the hormone response in transepithelial ion transport.

The effect of two-day treatment of primary cultured ovine somatotropes with GHRP-2 on membrane voltage-gated K+ currents

Long-term in vivo treatment with synthetic GH-releasing peptides (GHRPs) enhances the release of GH induced by endogenous GHRH. The mechanism for such an enhancement on GH release is unknown. In this experiment, somatotropes were obtained from ovine pituitaries by enzyme dissociation and enriched by density centrifugation. Membrane voltage and currents were recorded with whole-cell patch-clamp configuration. After 48-h treatment with GHRP-2 (10(-8) M), the percentage of cells with spontaneous action potential was increased (51 vs. 27%) without change of resting potential. This GHRP-2 treatment also increased the amplitude of voltage-gated K+ currents (predominantly transient A-type-like current but also delayed rectifier or K-type-like current) without modification of biophysical kinetics. Down-regulation of protein kinase C (PKC) with phorbol 12-myristate 13-acetate at the time of adding GHRP-2 blocked the increase in K+ currents. Inclusion of calphostin C (PKC inhibitor) but not H(89) (protein kinase A inhibitor) significantly reduced the increase in K+ currents by GHRP-2. Inclusion of actinomycin D (transcription inhibitor) or cycloheximide (protein synthesis inhibitor) abolished the increase in K+ currents. These data indicate that 48-h GHRP-2 treatment increases the density of K+ channels via PKC and channel protein synthesis. Such a modification on K+ channels by GHRP-2 may be partially responsible for the change of somatotrope electrophysiological properties and sensitivity to GHRH stimulation.

Transcriptome of a mouse kidney cortical collecting duct cell line: effects of aldosterone and vasopressin

Aldosterone and vasopressin are responsible for the final adjustment of sodium and water reabsorption in the kidney. In principal cells of the kidney cortical collecting duct (CCD), the integral response to aldosterone and the long-term functional effects of vasopressin depend on transcription. In this study, we analyzed the transcriptome of a highly differentiated mouse clonal CCD principal cell line (mpkCCD(cl4)) and the changes in the transcriptome induced by aldosterone and vasopressin. Serial analysis of gene expression (SAGE) was performed on untreated cells and on cells treated with either aldosterone or vasopressin for 4 h. The transcriptomes in these three experimental conditions were determined by sequencing 169,721 transcript tags from the corresponding SAGE libraries. Limiting the analysis to tags that occurred twice or more in the data set, 14,654 different transcripts were identified, 3,642 of which do not match known mouse sequences. Statistical comparison (at P < 0.05 level) of the three SAGE libraries revealed 34 AITs (aldosterone-induced transcripts), 29 ARTs (aldosterone-repressed transcripts), 48 VITs (vasopressin-induced transcripts) and 11 VRTs (vasopressin-repressed transcripts). A selection of the differentially-expressed, hormone-specific transcripts (5 VITs, 2 AITs and 1 ART) has been validated in the mpkCCD(cl4) cell line either by Northern blot hybridization or reverse transcription-PCR. The hepatocyte nuclear transcription factor HNF-3-alpha (VIT39), the receptor activity modifying protein RAMP3 (VIT48), and the glucocorticoid-induced leucine zipper protein (GILZ) (AIT28) are candidate proteins playing a role in physiological responses of this cell line to vasopressin and aldosterone.

Diverse intracellular signalling systems used by growth hormone-releasing hormone in regulating voltage-gated Ca2+ or K channels in pituitary somatotropes

Influx of Ca2+ via Ca2+ channels is the major step triggering exocytosis of pituitary somatotropes to release growth hormone (GH). Voltage-gated Ca2+ and K+ channels, the primary determinants of the influx of Ca2+, are regulated by GH-releasing hormone (GHRH) through G-protein-coupled intracellular signalling systems. Using whole-cell patch-clamp techniques, the changes of the Ca2+ and K+ currents in primary cultured ovine and human somatotropes were recorded. Growth hormone-releasing hormone (10 nmol/L) increased both L- and T-type voltage-gated Ca2+ currents. Inhibition of the cAMP/protein kinase A (PKA) pathway by either Rp-cAMP or H89 blocked this increase in both L- and T-type Ca2+ currents. Growth hormone-releasing hormone also decreased voltage-gated transient (IA) and delayed rectified (IK) K+ currents. Protein kinase C (PKC) inhibitors, such as calphostin C, chelerythrine or downregulation of PKC, blocked the effect of GHRH on K+ currents, whereas an acute activation of PKC by phorbol 12, 13-dibutyrate (1 micromol/L) mimicked the effect of GHRH. Intracellular dialysis of a specific PKC inhibitor (PKC19-36) also prevented the reduction in K+ currents by GHRH. It is therefore concluded that GHRH increases voltage-gated Ca2+ currents via cAMP/PKA, but decreases voltage-gated K+ currents via the PKC signalling system. The GHRH-induced alteration of Ca2+ and K+ currents augments the influx of Ca2+, leading to an increase in [Ca2+]i and the GH secretion.

Growth hormone-releasing hormone decreases voltage-gated potassium currents in GH4C1 cells

The electrophysiological properties of anterior pituitary somatotropes integrally involve the function of voltage-gated K+ currents. In this study, we have used GH4C1 cell lines to investigate the effect of human GHRH on voltage-gated K+ currents. Because of a clear 'rundown' of the K+ current with classic whole cell recording (WCR) without ATP in pipette solution, nystatin-perforated WCR was the major recording configuration used. Using a low Ca2+ (0.5 mM) bath solution containing Co2+ (1 mM) and TTX (1 microM), GH4 cells predominantly exhibited an outward delayed rectifier K+ current (IK). Local application of growth hormone releasing hormone (GHRH) (100 nM) reversibly reduced the amplitude of the K+ currents (to 83% of control). There was no effect of GHRH on the activation curve of the K+ current and no difference observed using 2.5 mM Ca2+ or low Ca2+ (0.5 mM Ca2++1 mM Co2+) bath solutions. Under the condition of low Ca2+ bath solution, the application of apamin (1 microM) or charybdotoxin (1 microM), two specific blockers of the Ca2+-activated K+ current, did not alter the K+ current or the response to GHRH. This reduction in the K+ current by GHRH was also observed with classic WCR with a pipette solution containing ATP (2 mM). The GHRH-induced reduction in the K+ current was completely abolished by the presence of GDP-beta-s (500 microM) in the pipette solution or by addition of PKC inhibitors, calphostin C (100 nM) and chelerythrine (1 microM), in bath solution. Inhibitor for cAMP-PKA system (Rp-cAMP and H89) did not affect the K+ current response to GHRH. These results suggest that GHRH reduces the voltage-gated K+ current in GH4C1 cells, a response that is mediated by G-proteins and PKC system but not by cAMP-PKA system. The reduction in the K+ current may partially contribute to the GHRH-stimulated growth hormone (GH) secretion.

Human GHRH reduces voltage-gated K+ currents via a non-cAMP-dependent but PKC-mediated pathway in human GH adenoma cells

1. Whole-cell voltage-gated K+ currents and the K+ current response to growth hormone-releasing hormone (GHRH) were characterised in primary cultures of human acromegalic somatotropes. 2. Both delayed rectifier (IK) and transient (IA) K+ currents were recorded from human somatotropes held at -80 mV and bathed in a solution containing Cd2+ (1 mM), TTX (1 microM) and a low concentration of Ca2+ (0.5 mM). Only IK was recorded, however, when a holding potential of -40 mV was used. 3. GHRH (10 nM) immediately and significantly reduced the amplitude of both IA and IK. While the reduction in the amplitude of IA was fully reversed following the removal of GHRH, the amplitude of IK had only partially recovered 10 min after GHRH removal. In addition, GHRH shifted the voltage-dependent inactivation curve of IA by 13.5 mV in the negative direction. 4. In a low Ca2+ and Cd2+-containing solution, the Ca2+-activated K+ channel blockers apamin (100 nM and 1 microM) and charybdotoxin (1 microM) did not alter K+ currents or the effect of GHRH on the recorded K+ currents. 5. The whole-cell K+ currents and their responses to GHRH were unaffected by the application of 8-bromo-cAMP (100 microM), Rp-cAMP (100 microM) or the protein kinase A (PKA) inhibitor H89 (1 microM). In addition, intracellular dialysis of the PKA inhibitory peptide PKI (10 microM) had no effect on the K+ current response to GHRH. 6. While the application of protein kinase C (PKC) inhibitors calphostin C (100 nM) or chelerythrine (1 microM) did not affect the amplitude of the K+ currents, the K+ current response to GHRH was significantly attenuated. Downregulation of PKC with phorbol 12,13-dibutyrate (PDBu, 0.5 microM for 16 h) also abolished the K+ current response to GHRH. In addition, intracellular dialysis of somatotropes with the PKC inhibitory peptide PKC19-36 (1 microM) prevented the GHRH-induced decrease in the amplitude of the voltage-gated K+ currents. Local application of PDBu (1 microM) significantly reduced the amplitude of the voltage-gated K+ currents in a similar manner to that induced by GHRH, but without clear recovery upon removal. 7. This study demonstrates that GHRH decreases voltage-gated K+ currents via a PKC-mediated pathway in human adenoma somatotropes, rather than by the cAMP-PKA pathway that is usually implicated in the actions of GHRH.

Gi-3 protein mediates the increase in voltage-gated K+ currents by somatostatin on cultured ovine somatotrophs

Voltage-gated K+ currents in rat somatotrophs are increased by somatostatin (SRIF) through unidentified G protein. In this experiment, somatotroph-enriched cells (up to 85%) were obtained from ovine pituitary glands and further identified by the increase in K+ currents by SRIF. The whole cell recording was employed to study the voltage-gated K+ currents. A reversible increase in K+ currents (up to 150% of control) was obtained in response to local application of SRIF (10 nM) but not vehicle. When the guanosine 5'-O-(3-thiotriphosphate) was included in the pipette solution (200 microM), the recovery phase of K+ current response to SRIF was abolished. Inclusion of guanosine 5'-O-(2-thiodiphosphate) (200 microM) in pipette solution blocked the K+ current response to SRIF. Intracellular dialysis of antibodies against alphao-, alphai-, alphai-1-2-, or alphai-3-subunits of G proteins via patch pipettes was confirmed by immunofluorescent staining of the antibodies. Antibody dialysis alone did not modify voltage-gated K+ currents. Dialysis of anti-alphai or anti-alphai-3 antibodies significantly attenuated the increase in K+ currents that was obtained after application of 10 or 100 nM SRIF. Dialysis with anti-alphao, anti-alphai-1-2, or heat-inactivated (60 degreesC for 10 min) anti-alphai antibodies did not diminish the effect of SRIF on K+ currents. We conclude that the Gi-3 protein mediates the effect of SRIF on voltage-gated K+ currents in ovine somatotrophs.

Regulation by growth hormone-releasing hormone and somatostatin of a Na+ current in the primary cultured rat somatotroph

The purpose of the present study is to characterize Na+ current activated by GH-releasing hormone (GHRH) and to investigate the effect of somatostatin (SRIF) on that current, because the Na+ current has been suggested to play a pivotal role in the process of GHRH-induced GH secretion. Primary-cultured pituitary somatotrophs were prepared from male Wistar rats. Whole-cell membrane currents were recorded and analyzed by a perforated patch clamp system. To isolate Na+ current, K+ and Ca2+ were replaced with Cs+ and Mg2+, respectively, in the extracellular solution, and cesium aspartate was used for the pipette solution. Furthermore, tetrodotoxin and nifedipine were added to the extracellular solution to eliminate the voltage-gated currents. Under these conditions, GHRH activated a mean inward Na+ current (-1.86 +/- 0.33 pA, mean +/- SE) at potentials between -50 and -20 mV and a smaller current (-0.59 +/- 0.13 pA) at potentials between -100 and -80 mV, which were completely blocked by protein kinase A blocker (H-89). In addition, SRIF (1-10 nM) partially suppressed these Na+ currents, which were not affected by phosphatase inhibitors (okadaic acid and calyculin A). These results suggest that GHRH activates the Na+ current through phosphorylation by protein kinase A and that SRIF partially suppressed this current and that the current was larger at more positive potentials than at more negative potentials.

G(o)2 and Gi3 proteins mediate the action of somatostatin on membrane Ca2+ and K+ currents in ovine pituitary somatotrophs

1. Growth hormone (GH) secretion from the anterior pituitary gland is mainly regulated by hypothalamic GH-releasing hormone (GHRH) and somatostatin (SRIF). Somatostatin reduces both spontaneous and GHRH-stimulated GH secretion. 2. Exocytosis of GH is mainly determined by the intracellular free Ca2+ concentration ([Ca2+]i), which is regulated by the influx of Ca2+ via membrane Ca2+ channels. Somatostatin reduces the influx of Ca2+ through two separate mechanisms, namely a direct action on Ca2+ channels and an indirect action on membrane potentials through the activation of K+ channels. 3. In the present experiments, somatotroph-enriched cells were obtained from the ovine pituitary gland by means of collagenase dissociation and Percoll-gradient centrifugation. Further identification was based on the effect of SRIF (10 nmol/L) on Ca2+ or K+ currents. 4. A significant reduction in Ca2+ currents and an increase in K+ currents was obtained in response to local application of SRIF (10 nmol/L), but vehicle application had no effect. The responses of Ca2+ and K+ currents to SRIF were reversible after removal of SRIF. 5. Dialysis of GTP-gamma-s (200 mumol/L) abolished the recovery phase of K+ current response to SRIF after its removal, whereas GDP-beta-s (200 mumol/L) totally blocked the response. Pretreatment of the cells with pertussis toxin (100 nmol/L) overnight abolished the Ca2+ current response to SRIF. 6. Intracellular dialysis of antibodies to alpha o, alpha i1-3, alpha i1-2 and alpha i3 subunits of the G-proteins into cells via whole-cell patch-clamp pipettes was confirmed by immunofluorescent staining of the antibodies. 7. Dialysis of anti-alpha i1-3 or anti-alpha i3 antibodies significantly attenuated the increase in the K+ current in response to 10 nmol/L SRIF, whereas neither anti-alpha o nor anti-alpha i1-2 antibodies diminished the effect of SRIF on the K+ current. 8. Dialysis of anti-alpha o antibodies significantly attenuated the reduction in the Ca2+ current that was obtained upon application of 10 nmol/L SRIF. Neither anti-alpha i1-2 nor anti-alpha i3 antibody dialysis diminished the effect of SRIF on the Ca2+ current. 9. Dialysis of the alpha o common antisense oligonucleotides (ASm) but not the alpha i3 AS significantly diminished the inhibitory effect of SRIF on the Ca2+ current. This effect of alpha o ASm dialysis occurred at 12 h incubation after dialysis, reaching a maximal level at 48 h and partially recovering at 72 h incubation. Antisense oligonucleotides specific for alpha o1 (alpha o1 AS) or alpha o2 (alpha o2 AS) were dialysed into somatotrophs and only alpha o2 AS significantly attenuated the inhibition of SRIF on the Ca2+ current. 10. It is concluded that the Gi3 protein mediates the effect of SRIF on the K+ current and that the G(o)2 protein mediates the effect of SRIF on the Ca2+ current in primary cultured ovine somatotrophs.

Effect on nociception of intracerebroventricular administration of low doses of neuropeptide Y in mice

The present study shows further evidence about the implication of neuropeptide Y (NPY) in nociception. The effect of NPY (1-36), when intracerebroventricularly administered, has been studied using two physical models of acute pain (hot plate test and electrical tail stimulation) and the formalin test. The animal response to these three pain models has been shown to be integrated at different levels in the CNS. A decrease in pain threshold was exhibited in both the hot plate test (10, 30, 60, 120 and 480 pmol of NPY i.c.v.) and the electrical tail simulation test (10, 30 and 60 pmol of NPY i.c.v.), while in the formalin test (10, 30, 60 and 120 pmol of NPY icv) the licking response decreased in phase I but not in phase 2. In these three tests NPY showed hyperalgesic or analgesic effects when administered at low doses, while at high doses it failed to induce any effect. Results show that the effect of NPY on nociception is clearly test-dependent and is only observed at low doses.

Withdrawal of somatostatin augments L-type Ca2+ current in primary cultured rat somatotrophs

It is known that withdrawal of somatostatin (SRIF) augments the growth hormone (GH) releasing hormone (GRF)-induced GH secretion. To investigate the mechanism of this augmentation in GH secretion, effects of GRF and SRIF on L-type Ca2+ current (Ba2+ was used as a charge carrier) or primary cultured rat somatotroph were studied by perforated patch clamp technique. The reason is that GRF-induced GH secretion is thought to be causally related to the influx of Ca2+ through L-type Ca2+ channels. 10 mM GRF augmented maximum amplitude of L-type Ba2+ current by 12.2% (n = 12). Subsequent application of SRIF slightly suppressed the currents but the suppression never exceeded the control level of the current. Removal of SRIF, however, promptly augmented the L-type Ba2+ current by 26.8%. Such off-response of SRIF was not observed in cells treated overnight with 100 ng/ml pertussis toxin. Further, specific inhibitor of protein kinase A, H-89 at 1 microM reversibly suppressed the augmentation of L-type Ba2+ current to control level. At 10 microM, H-89 suppressed L-type Ba2+ current by more than 40% from control level. These results suggest that (1) L-type Ca2+ channel of somatotroph is probably phosphorylated in a basal condition and may be slightly modulated by GRF through increased level of cAMP; (2) SRIF only slightly suppress the channel activity; (3) Withdrawal of SRIF facilitates the activity of L-type Ca2+ channel via PTX-sensitive G-protein, although the precise mechanism of this facilitation is unknown. The augmentation by SRIF-pretreatment of GRF-induced GH secretion may be at least partly due to the facilitation of the activity of L-type Ca2+ channel.

Modulation by somatostatin of glutamate sensitivity during development of mouse hypothalamic neurons in vitro

Glutamate sensitivity development and interactions of somatostatin (SRIF) with AMPA/Kainate receptor-mediated glutamate responses were studied in dissociated hypothalamic neurons from 16-day-old mouse embryos grown in vitro. Only 18% of functionally innervated cells could be found at 6-9 DIV whereas the percentage of innervated neurons progressively increased thereafter to reach 100% at 19-22 DIV. The glutamate sensitivity, estimated from glutamate-induced peak inward current, was very low at 6-9 DIV, sharply increased at 11-14 DIV and developed at a low increase rate thereafter. SRIF either unaffected glutamate peak current (27% of the cells), or significantly decreased (50%) or increased it (23%). Pertussis Toxin pretreatment abolished the SRIF-induced decrease of the glutamate response without affecting the excitatory effect. The number of glutamate responsive neurons inhibited by SRIF increased with time in culture whereas that of neurons responding to SRIF by an increased glutamate response was not statistically modified by functional innervation. The present data suggest that increased glutamate sensitivity coincides with the onset of functional synaptogenesis in mouse hypothalamic neurons in culture. SRIF can modulate glutamate sensitivity of hypothalamic neurons with either synergistic or antagonistic effects. Since glutamate has been shown to stimulate SRIF synthesis and secretion from hypothalamic neurons, the reverse capacity of SRIF to modulate the glutamate response suggests that both transmitters exhibit complex reciprocal interactions.

Effect of nonpeptide vasopressin receptor antagonists on developing, and established DOCA-salt hypertension in rats

Efficacy of orally available, selective vasopressin V1 and V2 receptor antagonists on the developing and established stage of DOCA-salt hypertension was investigated. Twenty-nine Wistar rats were heminephrectomized, and administered DOCA (50 mg/kg; intraperitoneally twice a week) and salt (5% NaCl diet) from week 0 to the end of study. Group 1 rats were served as control. Group 2 and 5 rats were given a V1 antagonist, and groups 3 and 6 rats were given a V2 antagonist, while groups 4 and 7 rats received both V1 and V2 antagonists. Each drug was started to groups 2, 3 and 4 at week 0, and to groups 5, 6 and 7 at week 4. Significant amelioration of the increase in blood pressure was observed in groups 3 and 4 at week 10, and a reduction in blood pressure occurred in groups 6 and 7 at week 10. Otherwise, a V1 antagonist alone slightly attenuated blood pressure rise in the group 2 without significance, and failed to lower blood pressure of the group 5 during the study. These results suggest that both V1 and V2 agonisms are involved in an increase in blood pressure at the developing stage of DOCA-salt hypertension, and that V2 agonism, but not V1 plays a major role in the maintenance of high blood pressure at the established stage.

Ion channels and the signal transduction pathways in the regulation of growth hormone secretion

The secretion of GH from pituitary somatotrophs is mainly regulated by alterations in the levels of intracellular free Ca(2+) concentrations ([Ca(2+)](i)) that depend on the influx of Ca(2+) through voltage-gated Ca(2+) channels in the cell membrane. Hypothalamic stimulatory and inhibitory factors bind to specific receptors on the cell membrane to regulate membrane potential and activate second-messenger systems. The receptors are G-protein coupled, and activated G proteins directly influence membrane ion channels to regulate Ca(2+) influx. The function of cAMP-dependent protein kinase A is also modulated by receptor-coupled G proteins leading to the phosphorylation of Ca(2+) channel proteins and further alteration of Ca(2+) influx.

ATP-modulated K+ channels sensitive to antidiabetic sulfonylureas are present in adenohypophysis and are involved in growth hormone release

The adenohypophysis contains high-affinity binding sites for antidiabetic sulfonylureas that are specific blockers of ATP-sensitive K+ channels. The binding protein has a M(r) of 145,000 +/- 5000. The presence of ATP-sensitive K+ channels (26 pS) has been demonstrated by electrophysiological techniques. Intracellular perfusion of adenohypophysis cells with an ATP-free medium to activate ATP-sensitive K+ channels induces a large hyperpolarization (approximately 30 mV) that is antagonized by antidiabetic sulfonylureas. Diazoxide opens ATP-sensitive K+ channels in adenohypophysis cells as it does in pancreatic beta cells and also induces a hyperpolarization (approximately 30 mV) that is also suppressed by antidiabetic sulfonylureas. As in pancreatic beta cells, glucose and antidiabetic sulfonylureas depolarize the adenohypophysis cells and thereby indirectly increase Ca2+ influx through L-type Ca2+ channels. The K+ channel opener diazoxide has an opposite effect. Opening ATP-sensitive K+ channels inhibits growth hormone secretion and this inhibition is eliminated by antidiabetic sulfonylureas.

Actions of somatostatin on GABA-ergic synaptic transmission in the CA1 area of the hippocampus

Somatostatin and gamma-aminobutyric acid (GABA) are co-localized in some neurons in the CA1 area of the hippocampus. Since it is possible that the peptide and the amino acid are co-released, the interactions between the actions of somatostatin and GABA-ergic inhibitory post-synaptic potentials (IPSPs) in the CA1 pyramidal neurons of guinea pig hippocampal slices have been investigated. Somatostatin (2 microM) induced a hyperpolarization of the CA1 neurons associated with a reduction in the input resistance of the cells. These effects were not blocked by picrotoxinin (20 microM) or phaclofen (1 mM). Chelation of intracellular Ca2+ (Ca2+i) with BAPTA or the inhibition of protein kinase C (PKC) with sphingosine (30 microM) had no significant effects on the hyperpolarizing actions of somatostatin. The peptide suppressed the GABAA receptor-mediated fast IPSPs and the GABAB receptor-mediated slow IPSPs, but had no significant effect on the excitatory post-synaptic potentials (EPSPs). Somatostatin-induced depression of the IPSPs was not due to the hyperpolarization of the neurons. Baclofen (20 microM) suppressed the EPSP, as well as the fast and the slow IPSPs. The hyperpolarization of the CA1 neurons caused by somatostatin was greatly reduced in the presence of baclofen, an effect that was not due to the hyperpolarization of the cell by baclofen. The presence of QX-314 in the CA1 neurons, which suppressed the Na+ spikes and the slow IPSPs, prevented the hyperpolarization of the neurons by somatostatin and baclofen.(ABSTRACT TRUNCATED AT 250 WORDS)

Effectors of ATP-sensitive K+ channels inhibit the regulatory effects of somatostatin and GH-releasing factor on growth hormone secretion

Somatostatin inhibition of growth hormone (GH) secretion from adenohypophysis cells in culture was antagonized by the antidiabetic sulfonylurea glipizide (K0.5 = 10 +/- 5 nM). Although all cells that hyperpolarize with somatostatin have ATP-sensitive K+ channels, the antagonistic actions of the hormone and of the antidiabetic drug are due to effects on different types of K+ channels. Diazoxide, an opener of ATP-sensitive K+ channels, abolished the increase of intracellular Ca2+ provoked by growth hormone releasing factor (GRF) and induced inhibition of GRF stimulated GH secretion (K0.5 = 138 microM). This inhibition by diazoxide was largely suppressed by glipizide which blocked the ATP-sensitive K+ channels opened by diazoxide. In summary, hormonal activation of GH secretion is inhibited by openers of ATP-sensitive K+ channels, while hormonal inhibition of GH secretion is suppressed by blockers of ATP-sensitive K+ channels.

Ion conductances of isolated cortical collecting duct cells

The study of ion conductances in the intact cortical collecting duct (CCD) with the patch-clamp method is rather difficult. An optimized method to isolate CCD cells from rat kidneys using an in vivo followed by an in vitro enzyme digestion is described. Individual CCD segments were collected after this digestion and incubated in EGTA-buffered medium. This procedure resulted in single cells or cell clusters. These freshly isolated CCD cells were studied with different modifications of the patch-clamp method. Membrane voltages measured in the cell-attached-nystatin configuration were -74 +/- 1 mV (n = 13) and -68 +/- 3 mV (n = 22) in cells isolated from normal and mineralocorticoid-treated rats respectively. These values and those measured with the nystatin-perforated slow-whole-cell configuration (-79 +/- 1 mV, n = 23) are comparable to those measured in principal cells of isolated CCD segments. The cells hyperpolarized after the addition of amiloride and depolarized with the addition of adiuretin to the bath. The amiloride effect was enhanced when cells were isolated from deoxycorticosterone-acetate-treated rats. The cells were strongly depolarized upon elevation of the extracellular K(+)-concentration and did not demonstrate a measurable Cl- conductance. A large-conductance K+ channel (174 pS, n = 5, cell-attached, 145 mmol/l K+ in the pipette; 140 pS, n = 12, cell-free, 3.6 mmol/l K+ in the bath) was seen. It had a very low activity on the cell, but a high open probability when excised into a solution with 1 mmol/l Ca2+ on the cytosolic side.(ABSTRACT TRUNCATED AT 250 WORDS)

Somatostatin activates an inwardly rectifying K+ conductance in freshly dispersed rat somatotrophs

1. Somatotrophs from enzymatically dispersed anterior pituitary glands of rats, enriched to greater than 94% purity by density gradient centrifugation, were studied within 16 h of isolation using patch clamp recording methods in the conventional whole-cell and the perforated-patch configurations. 2. Rhythmic oscillations of membrane potential gave rise to action potentials in thirty-six of fifty-two cells studied with the perforated-patch technique. Membrane potential oscillated between approximately -70 mV and approximately -25 mV with an average frequency (mean +/- S.D.) of 0.9 +/- 0.9 s-1. 3. The current-voltage (I-V) relationship of cells was linear at negative potentials with outward rectification at potentials positive to -40 mV. Evidence that the outward current was due to K+ channels came from the deactivation tail currents, which reversed direction close to the K+ equilibrium potential (EK). The reversal potential shifted 60 mV per tenfold change of external K+ concentration ([K+]o), as expected for K+ current. 4. Suppression of outward current by tetraethylammonium (TEA) provided additional evidence for K+ current. Cd2+ reduced outward current, suggesting the presence of Ca(2+)-activated K+ conductance. 5. Depolarizing commands elicited transient inward Na+ current and a sustained Ca2+ current (ICa). ICa was recorded in isolation with Cs+ and TEA in the recording pipette and 10 mM-Ba2+ as the charge carrier. Activation of ICa began at approximately -40 mV, with peak inward current at 0 to +10 mV. The half-inactivation potential was approximately -35 mV. In addition, ICa was blocked by nifedipine. These characteristics indicate the presence of L-type Ca2+ channels in somatotrophs. 6. Somatostatin caused hyperpolarization and suppressed the spontaneous bursts of action potentials. Under voltage clamp, somatostatin activated an inwardly rectifying current that reversed direction near EK. When EK was altered by elevation of [K+]o, the reversal potential of the somatostatin-induced current shifted 55 mV per tenfold change of [K+]o, as predicted for a K+ current by the Nernst relation. The somatostatin-induced conductance (gK) was greater at more negative potentials, and the activation range shifted positive with elevation of [K+]o. 7. We conclude that freshly isolated rat somatotrophs possess Na+, Ca2+ and K+ currents. A large proportion of the cells exhibit spontaneous bursts of action potentials. Somatostatin activates an inwardly rectifying K+ conductance, causing hyperpolarization and cessation of spontaneous action potential activity, actions that would contribute to suppression of growth hormone release.